The technical portion of the optician's profession consists in mounting a pair of ophthalmic lenses on the frame selected by the wearer. Such mounting comprises two main operations:                centering each lens, which consists in appropriately positioning the lens in register with the eye of the future wearer, and consequently in register with the frame; and then        edging each lens, which is an operation consisting in machining or cutting out the outline of the lens to a desired shape, while taking account of the defined centering parameters.        
In the context of the present invention, attention is given to the first operation of centering, specifically on the assumption that the frame is of the rimless type. Specifically, the optician needs to define the position that the optical frame of reference of an ophthalmic lens for eyeglasses (typically as obtained from the usual marks or from its optical center) is to occupy on a frame of known shape, so that the lens is suitably positioned in register with the corresponding eye of the wearer in order to perform the optical function for which it has been designed as well as possible.
To do this, the optician initially situates the position of the pupil of each eye on the corresponding lens. This thus determines two main parameters associated with the morphology of the wearer, namely the pupillary distance or the two pupillary half-distances defined as the distances between each of the pupils of the two eyes and the nose of the wearer, and also the heights of the pupils relative to the frame fitted on the wearer's face. When the frame is of the type having rims, the heights may be the so-called “under-pupil” or “datum” heights, defined as being the distance between the pupil of the eye in question and the point of the frame that is situated vertically below the pupil of the eye. In analogous manner, when the frame is of the rimless type, the under-pupil height is defined as being the distance between the pupil of the eye in question and the point on the outline of the lens that is situated vertically below the pupil of the eye. In practice, the optician measures the height as defined in this way on a template-forming pair of presentation lenses constituted by a frame and lenses that are identical in shape to the shape of the lenses that are to be made.
Thereafter, the optician transfers the height as measured in this way to the lens for edging, starting from the position of the centering point of said lens as previously identified and that needs to be positioned relative to the pupil of the wearer. This transferred height together with the corresponding pupillary half-distance then enables the lens to be centered, i.e. to be properly positioned on the frame so as to properly position the desired edging outline on the lens.
More precisely, the centering method presently in use for rimmed and for rimless frames consists in:
i) fitting on the wearer presentation eyeglasses comprising a presentation frame identical to said target frame and at least one presentation lens associated with the same eye as said target lens, and presenting an outline of shape and position relative to the frame that are identical to the shape and position intended for the final outline of the target lens after edging;
ii) pointing out manually, or identifying automatically, on the presentation lens the pupillary point situated in register with the pupil of the corresponding eye of the wearer;
iii) measuring the pupillary height of the pupillary point of the presentation lens;
iv) measuring the pupillary distance or half-distance of the wearer;
v) using deflectometry, interferometry, or image recognition to identify the optical frame of reference of the target lens, and in particular the centering point of the target lens that is to be placed in register with the pupil of the wearer when the wearer is wearing the target eyeglasses;
vi) transferring the pupillary height and the pupillary distance or half-distance onto the target lens from said centering point of the target lens; and
vii) deducing therefrom the centering position of the target lens relative to the target frame.
In practice, step iii) is performed by measuring the height difference between the pupillary point of the presentation lens and a presentation reference point associated with the presentation lens. The presentation reference point associated with the presentation lens may be the lowest point of the presentation frame, for example.
Thereafter, in step vi), this height difference is transferred onto the target lens starting from its centering point. Thereafter, in step vii), the centering position of the target lens relative to the target frame, or vice versa, is deduced so that a target remarkable point of the target lens corresponding to the remarkable point of the presentation lens presents the same height difference relative to the centering point as the difference between the remarkable point of the presentation lens and the pupillary point.
It can thus be understood that with that method, it is necessary for the presentation lens to present an outline of shape identical to the shape intended for the final outline of the target lens after edging.
However, it is found that consumers are tending more and more towards treatment that is personalized to a greater extent. With rimless frames, for reasons of fashion or visual comfort, it is then desirable to have great freedom in determining the shape of the lens outline. Unfortunately, the present method of taking pupil height measurements with reference to the bottom edge of a template makes it necessary to have a template of shape identical to the shape that it is desired to obtain finally for the lens. In other words, it is necessary to have as many templates as there are variations in lens shape, which is particularly inconvenient and inflexible, and also restrictive on the freedom that wearers can be given in selecting lens shapes.